1. Field of the Invention
The present invention relates to an optical fiber cable wound about an overhead transmission line for transmitting power of high voltage, said optical fiber cable being used for optical communication or the like.
2. Description of the Related Art
An optical fiber is not electrically affected by a power line such as a transmission line. Based on this property, an optical fiber cable B wound about an overhead transmission line A installed in advance has been developed as shown in FIG. 1 to permit communication between, for example, a power station and a substation.
The optical fiber cable B wound about the overhead transmission line A is required to meet the conditions described below. First of all, the optical fiber cable is heated by the heat generated from the overhead transmission line itself. In, particularly, the summer season, the overhead transmission line is heated to temperatures exceeding 100.degree. C. in some cases, with the result that the optical fiber cable is also heated to about 100.degree. C. If an accident occurs by short-circuiting in the overhead transmission line, the optical fiber cable is momentarily heated to about 300.degree. C. Naturally, the optical fiber cable wound about the overhead transmission line is required to exhibit a high heat resistance, i.e., resistance to about 300.degree. C.
It should also be noted that an overhead transmission line is stretched between supporting poles installed a predetermined distance apart from each other and, thus, receives a very high wind pressure. Naturally, it is important for the optical fiber cable wound about the overhead transmission line to have an outer diameter as small as possible. To be more specific, the outer diameter of the optical fiber cable should be at most about 5 mm.
A conventional optical fiber cable constructed to meet the two requirements described above has a cross section as shown in FIG. 3. It is seen that a plurality of single-core type optical fiber strands 16, each having a covering, are arranged about the outer surface of a tension member 1, made of, for example, FRP (Fiber Reinforced Plastic). Further, the optical fiber strands 16 are covered with a fluoroplastic sheath (protective layer) 17. FIG. 2 shows the construction of the single-core type optical fiber strand 16. Specifically, the strand 16 comprises an optical fiber 13 consisting of a core 11 and a clad 12 each formed of silica glass, a buffer layer 14 made of, for example, a silicone resin and positioned to cover the optical fiber 13, and an outermost layer 15 made of, for example, a fluoroplastic and positioned to cover the buffer layer 14.
In the conventional optical fiber cable described above, the outermost layer 15 of the single-core type optical fiber strand 16 is made of a fluoroplastic. Since the fluoroplastic outermost layer 15 is hard and permits sufficiently protecting the optical fiber 13, it is unnecessary to provide a buffer layer made of, for example, a silicone resin to cover the outer surface of the tension member 1. However, in the case of preparing an optical fiber cable having an outer diameter of about 5 mm by using the single-core type optical fiber strands 16, only about 12 strands can be incorporated in the optical fiber cable. An optical fiber cable having only about 12 single-core type optical fiber strands incorporated therein is incapable of handling an increase in data expected in the future. In other words, the optical fiber cable is incapable of transmitting a large amount of optical signals.
For enabling the optical fiber cable to be capable of transmitting a large amount of optical signals, it is certainly possible to incorporate a larger number of the single-core type optical fiber strands 16 by diminishing the outer diameter of each of the strands 16. As a matter of fact, the outer diameter of the strand 16 can be diminished by decreasing the thickness of the outermost layer 15 of the strand 16. It is also possible to diminish the outer diameter of the optical fiber cable by decreasing the thickness of the protective layer 17 to permit the cable of a predetermined thickness to include a larger number of the single-core type optical fiber strands 16. If the thickness of the outermost layer 15 or the protective layer 17 is decreased, however, the optical fiber cable fails to exhibit a sufficient heat resistance and satisfactory mechanical properties such as tensile strength and so on.